Approximately 99% of body calcium is in bone, most of the remaining 1% is in extracellular fluid. Calcium is an important regulator of many cellular functions in the body, including neuromuscular activity, blood coagulation, intercellular signal transmission, exocrine-endocrine functions and the control of cell membrane potentials.
Calcium balance is regulated by parathyroid hormone and calcitriol.
The cell- surface-calcium-sensing receptor (CASR) is expressed abundantly in the parathyroid hormone or PTH producing chief cells of the parathyroid gland, the calcitonin-producing C-cells of the thyroid and the cells lining the kidney tubule. It increases serum calcium by stimulating bone resorption, increasing reclamation in the kidney and promoting renal conversion of vitamin D to calcitriol.
Calcitriol is the active form of vitamin D and it stimulates intestinal absorption of calcium and is one of many factors that provide feedback to the parathyroid gland.
Calcium is absorbed in the proximal tubules though a solvent gradient but active transport occurs in the distal portions by a sodium calcium exchange pump. Some calcium absorption also takes place in the loop of Henle via an electrochemical gradient.
Calcium homeostasis is important in critically ill patients to prevent blood pressure and cardiac instabilities.
Calcium homeostasis
